1. Field of the Invention
The present invention relates to an apparatus for automatic gain control (AGC) in a receiver of a wireless communication system.
2. Discussion of Related Art
In general, the output power of a transmission unit in a wireless communication system is uniform, but the amount of signal gain varies according to movement of a receiver, distance of the receiver, and channel environment.
However, in order to ensure a sufficient dynamic range of an analog-digital (AD) converter, inputs of the AD converter must have a uniform level. Therefore, the gain of a receiver must vary to compensate for change in distance or channel environment.
Such an operation is performed by an apparatus for AGC. The AGC apparatus compares the power of an input signal with a reference power and determines the gain of the input signal. Here, the gains of a radio frequency (RF) amplifier and an intermediate frequency (IF) amplifier are controlled according to the determined gain, to amplify the signal to a desired level.
As illustrated in FIG. 1, most AGC apparatuses that are currently used adjust the gains of an RF amplifier 100 and IF amplifier 103 using gain control signals generated by a digital AGC 105 of a digital demodulation unit 104. The digital demodulation unit 104 also includes an analog/digital (ND) converter 106.
In FIG. 1, a modulated RF signal is input to a receiver through an antenna, and the RF amplifier 100 receives a gain control signal and amplifies the input signal. Since the amplified RF signal is generally a high-frequency signal, a large amount of power is needed to process it and filtering is difficult. Thus, the amplified RF signal is down-converted into an IF signal by a mixer 101 of FIG. 1 and then passed through a band-pass filter 102 to remove a part of the signal outside a channel. In order to supplement insufficient gain of the RF amplifier 100, the IF amplifier 103 amplifies the IF signal. Here, the IF amplifier 103 receives an AGC signal of the digital demodulation unit 104 and adjusts gain.
When the AGC signal is received to adjust gain in a digital domain, linearity of the gains of the RF amplifier 100 and the IF amplifier 103 must be ensured. When the linearity of the gains of the amplifiers is not ensured, the stability of the AGC loop deteriorates.
In addition, in order to control gain in a digital domain, characteristics of devices in an analog domain must be considered, and so design becomes more difficult. More specifically, an AGC loop gets longer, and consequently the number of design parameters to be considered increases.
A conventional AGC loop of an analog domain is set up such that an RF AGC loop and an IF AGC loop operate separately so as to ensure stability. However, the RF amplifier 100 is generally designed to have a step gain in consideration of linearity and a noise characteristic. Therefore, it is hard to use an AGC loop, and thus the gain of the RF amplifier 100 is controlled by applying a received-signal-strength-indicator (RSSI). In this case, however, an RSSI block must process high-frequency signals, thus consuming a large amount of power.